Properties and Challenges of Thermally Treated Biomass by Dry and Wet Torrefaction

The torrefied solid biofuels have come to attract attention as promising alternative fuels to coal in coal-fired power plants and industrial boilers. The torrefaction is a biomass thermal treatment conducted at mild pyrolysis temperatures and includes two distinct technologies, dry torrefaction (DT) and wet torrefaction (WT). The DT is the conventional thermal treatment conducted in the absence of oxygen under atmospheric pressure in a temperature range of 200-300℃, while the WT is a thermal treatment in hydrothermal media or hot compressed water in a temperature range of 180-260℃. Both torrefaction treatments can produce energy dense solid biofuels with better chemical, physical properties than original biomass feedstock, but recently the research activities in WT have increased due to some advantages of WT over DT. In this article, properties and challenges of thermally treated biomass by dry and wet torrefaction are summarized. Finally, comparison of energy required in torrefaction process between DT and WT is also discussed.